Material handling equipment, such as carts, are used to transport a variety of objects. One example of a cart is a shopping cart used to transport groceries in the store and to and from vehicles. Another example of a cart is a utility cart used to transport dry goods such as lumber, and other kinds of bulk materials. Still another kind of cart is a luggage cart, a baker's rack, and so on.
The carts roll on wheels. The wheels are typically provided with a rubber or plastic tread, which tread is non-conductive.
When the cart moves across a surface, such as a store floor, the cart picks up an electrical charge in low humidity environments. The phenomena is similar to a person in socks or rubber sole shoes shuffling on a carpet in a low humidity environment. When the person touches a door knob or other metal object that is grounded, a static discharge occurs and the person feels “shocked”. Similarly, with a cart, as the cart becomes electrically charged, due to its electrical isolation from the ground, it can be discharged through a person, resulting in an unpleasant electrical shock. The shock occurs when the person touches the cart through grounded shoes or touches the cart and the grounded object such as a metal shelf or counter.
It is desirable to make the cart self-grounding in order to avoid the build-up of an electrical charge and the discharge thereof. A self-grounded cart has an electric path from the cart to the ground. This electric path can be separate from the wheels or can be through the wheels.
In the prior art, electrical paths are provided by a metal chain that is separate from the wheel. An example is shown by Lissner, U.S. Pat. No. 4,934,022. The chain extends from the metallic body or chassis of the cart to the ground. The chain is typically located adjacent to a wheel. The chain picks up debris as it drags along the ground. This debris may cause the chain to lose its electrical connection to the ground.
Still another way to ground the cart is directly through the wheels. Various prior art patents accomplish this, such as U.S. Pat. Nos. 4,716,997 and 6,786,559, as well as Swiss Patent No. 643,189.
It is desirable to have a low cost wheel that is reliable and that provides a conductive path to the ground.